Onset of acetylcholine sensitivity and endplate activity in developing myotome muscles of Xenopus

THE distribution of acetylcholine receptors and the onset of functional innervation at the neuromuscular junction during development have been much investigated^1-3, as have the events during regeneration and reinnervation of muscle⁴. Although the first appearance of acetylcholine (ACh) sensitivity of the membrane has been charted in chick and mouse myoblasts differentiating in tissue culture⁵, previous studies in vivo have either not tested for the initial appearance of ACh sensitivity², or found the muscle membrane to be already sensitive to externally applied ACh¹. In the present study on Xenopus embryos we have followed the initial appearance of membrane depolarisation in response to applied ACh and the onset of functional innervation in differentiating myotome muscle in vivo.